Integrated circuit with a test circuit

ABSTRACT

An integrated circuit with a test circuit having a measurement converter circuit and an activation unit. The measurement converter circuit converts one or more circuit-internal signals into a measured value. The activation unit activates the measurement converter circuit in accordance with an activation signal. The measurement converter circuit and the activation unit are connected to a connection pad. The activation unit is configured in such a way as to switch on the measurement converter circuit by means of the activation signal received via the connection pad. The measured value can be tapped off via the connection pad.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority benefits under 35 U.S.C. §119to co-pending German patent application number 103 14 616.4, filed Apr.1, 2003. This related patent application is herein incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an integrated circuit with a test circuit forchecking circuit-internal signals with the aid of an external testerdevice. The invention furthermore relates to a method for checking afunction in an integrated circuit. The invention furthermore relates toa test system with an integrated circuit and a tester device forchecking a function in the integrated circuit.

2. Description of the Related Art

During the testing of integrated circuits, it is necessary to detectinternal signals and forward them to a tester device for checking. Theinternal signals can be tapped off on the one hand via connection padsof the integrated circuit or by placing measuring tips on connectinglines of the integrated circuit. The number of available connection padsis limited on an integrated circuit since said connection pads take up alarge area in comparison with the circuit structures. On the other hand,signals detected by measuring tips are measurable only with a largeerror tolerance being accepted, since the driver power of the signalsbetween the gate circuits of the integrated circuit usually does notsuffice to sufficiently compensate for the inductive load of a measuringtip or of the contact between measuring tip and interconnect. Equally,in the case of analog signals, the resistance of the measuring tip, ofthe tester line with respect to the measuring tip and/or of the contactbetween measuring tip and interconnect is beset with an unknownresistance which corrupts the measurement of currents or voltages.

In the case of very weak signals, in particular, the measurement with anexternal tester device leads to unusable results, irrespective ofwhether the measured signal is measured via a connection pad or byemplacement of a measuring tip.

Hitherto, test mode circuits have been provided in the integratedcircuit, which check the internal signals or internal signalrelationships and compare them with corresponding desired values inorder to check the circuit in accordance with its specification. Onedisadvantage of the test mode circuits is the large space requirement inthe integrated circuit, since each test mode circuit has to be connectedto a control circuit with an interconnect.

A further disadvantage of measuring internal signals by means of anexternally connected tester device is that the signal changes in someinstances take place so rapidly that the external tester device cannotascertain them on account of capacitances and inductances.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide anintegrated circuit in such a way that internal signals can be checked ina simple and accurate manner and with a smallest possible number ofconnection pads used. Furthermore, it is an object of the presentinvention to provide a method for checking a function in an integratedcircuit, it being possible to check internal signals in a simple mannerwith the aid of a tester device.

A first aspect of the present invention provides an integrated circuitwith a test circuit. The test circuit has a measurement convertercircuit for converting one or more circuit-internal signals into ameasured value and an activation unit for activating the measurementconverter circuit in accordance with an activation signal. Themeasurement converter circuit and the activation unit are connected to aconnection pad. The activation unit is configured in such a way as topermanently switch on the measurement converter circuit by means of theactivation signal received via the connection pad, it being possibleafter switch-on to tap off the measured value via the connection pad.

The integrated circuit according to the invention thus makes it possibleto use an arbitrary connection pad of the integrated circuit forchecking internal signals. The internal signals are conditioned with theaid of a measurement converter circuit in such a way that they can bemeasured in a simple manner with the aid of a tester device. In thiscase, the signals are conditioned or converted or amplified in such away that possible contact-making or conduction resistances between themeasuring circuit and the tester device have as little influence aspossible on the measurement of the measured value generated by themeasurement converter circuit. Since the measurement converter circuitis essentially switched off in the unactivated state, the currentrequirement is not increased by the measurement converter circuitsduring normal operation of the integrated circuit. The use of themeasurement converter circuit makes it possible to convert weak signalswhich are driven internally within the circuit only with a weak driverpower in such a way that they can be measured externally in a simplemanner and with the least possible error. Moreover, such a measurementconverter circuit can convert rapid signal changes into a measured valuein such a way that the measured value makes it possible to check thesignal on the basis of a desired value of the measured value.

The invention has the advantage that the additional measurementconverter circuit does not significantly influence the signals examined,or does not influence them at all, so that the checked function and thecircuit-internal timing are not altered as a result. Since themeasurement converter circuit or the activation unit is preferablyarranged locally in the vicinity of the circuit of the checkingfunction, the complexity of the overall circuit is not significantlyincreased, as is the case for example when providing test mode circuitswhich are driven by a common control logic.

It may preferably be provided that the activation unit has an SRflip-flop and a switching element. The switching element, with the SRflip-flop not set, is switched in such a way as to connect theconnection pad to a predetermined potential via a specific resistor. Theconnection pad is connected to a set input of the SR flip-flop in orderto set the flip-flop by means of the activation signal in such a waythat a switching element is switched in such a way as to isolate theconnection pad from the predetermined potential. The use of an SRflip-flop in the activation unit makes it possible, e.g. by means of asetting of the SR flip-flop, to establish a permanent state in which theconnection pad can be used as a test connection. In the state in whichthe connection pad is not used as a test connection, the connection padis preferably connected to the predetermined potential, often a groundpotential, via a defined resistor, so that it is possible to avoidfloating of the connection pad and thus an undesired activation of theSR flip-flop.

In one embodiment, the switching element is preferably formed as afield-effect transistor.

Preferably, a reset input of the SR flip-flop is provided in order, inthe case of a reset signal and/or a switch-on signal of the integratedcircuit, to reset the SR flip-flop and to deactivate the measurementconverter circuit. This makes it possible, after the conclusion of thechecking of the one or more circuit-internal signals, to deactivate themeasurement converter circuit, so that it does not consume a currentduring normal operation.

Preferably, the measurement converter circuit has a phase comparisonunit, which compares the phase angle of a first and a second periodicsignal with one another and outputs a pulse-width-modulated signaldepending on the phase angle. In this way, it is possible to check thephase angle of two periodic signals that are related to one another,without the two signals firstly having to be tapped off by means of anexternal tester device in order to compare them with one another in theexternal tester device. The generation of the pulse-width-modulatedsignal in the integrated circuit thus makes it possible to carry out amore accurate determination of the phase angle, the measured value beingconfigured in such a way that it can be read out by the external testerdevice simply and as accurately as possible.

The measurement converter circuit may also have a digital-to-analogconverter unit in order to convert a digital signal into an analogvoltage value, it being possible to output the analog voltage value as ameasured value. The outputting of the measured value as an analog(essentially constant) voltage value has the advantage that thecapacitances and inductances between the tester device and themeasurement converter circuit essentially have no influence on theaccuracy of the measurement.

A further aspect of the present invention provides a test system with atester device and the integrated circuit according to the invention. Thetester device is connected to the integrated circuit via a testerchannel in order to transmit an activation signal to the integratedcircuit and, after the transmission of the activation signal, to receivethe measured value. Preferably, the tester device compares the receivedmeasured value with a desired measured value in order to check afunction of the integrated circuit.

A further aspect of the present invention provides a method for checkinga function in an integrated circuit. An activation signal is applied toa connection pad of the integrated circuit. The application of theactivation signal is followed by a measurement of one or morecircuit-internal signals, dependent on the function, in the integratedcircuit. A resulting measured value is read out via the connection pad,the measured value depending on the checked function.

The method according to the invention has the advantage that a measuredvalue which depends on the one or more circuit-internal signals isalready generated in the integrated circuit, and that the measured valueis read out via the connection pad. As a result, it is possible tomeasure very weak signals or operations in the case of very fastsignals.

The activation signal is preferably a pulse signal which is applied tothe connection pad in order to store an information item, themeasurement being carried out depending on the storage of theinformation item.

A phase signal is preferably read out as a measured value, which phasesignal is obtained by a comparison of two periodic signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with reference to theaccompanying drawings, in which:

FIG. 1 shows a block diagram of an integrated circuit with a measurementconverter circuit in accordance with an embodiment of the invention;

FIG. 2 shows a possible embodiment of an activation unit;

FIG. 3 shows a possible embodiment of a measurement converter circuit;and

FIG. 4 shows a signal profile for input signals and the output signal inaccordance with the measurement converter circuit according to FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a block diagram of an integrated circuit 1 accordingto the invention with a test circuit 2. The test circuit has ameasurement converter circuit 3 and an activation unit 4. The activationunit 4 is connected to a connection pad 6 via a first connection 5.Equally, the measurement converter circuit 3 is connected to theconnection pad 6 via a second connection 7. The activation unit 4 isconnected to the measurement converter circuit 3 via a switch-on line 8in order to transmit a switch-on signal EN to the measurement convertercircuit 3.

The activation unit 4 is configured in such a way as to receive, via theconnection pad 6, an activation signal which can be generated by anexternal tester device 9. The activation unit 4 has a memory unit 15, towhich the activation signal is written in such a way that a switch 10likewise provided in the activation unit 4 is opened. The switch 10 isconnected in series with a resistor 11, so that the connection pad 6,with switch 10 closed, is connected to a fixed potential, preferably aground potential, via the resistor 11. The switch 10 is closed after aninitialization and/or after a switch-on of the integrated circuit 1 andis opened as a consequence of the activation signal.

The switch-on signal EN is also made available to the measurementconverter circuit 3 via the switch-on line 8. The measurement convertercircuit 3 is switched on by the switch-on signal EN and starts toconvert measured internal signals.

For this purpose, the measurement converter circuit 3 is connected to afirst internal signal via a first signal line 12 and to a secondinternal signal via a second signal line 13. In the exemplary embodimentshown, the measurement converter circuit 3 has the function ofconverting the phase angle of the first and the second internal signalS1, S2 into a measured value M, which can be measured by the externaltester device 9 via the connection pad 6. The measured value M may be avoltage signal or a current signal which is converted into aproportional voltage via the resistor 11. The measured value may also bea pulsed signal which is output with a sufficient driver power to theconnection pad 6, and the frequency of which is influenced only to aninsignificant extent by line inductances and capacitances. The first andsecond internal signals S1, S2 are taken from a circuit 14, for example,in order to be able to check the function of the circuit. In this case,the first and second internal signals S1, S2 are tapped off in such away that the circuit 14 or the function thereof is influenced as littleas possible.

The measurement converter circuit 3 may be formed in diverse ways. Adigital-to-analog converter circuit may be involved, which outputs avoltage value as a measured value. Internal states can thus be codedinto an analog signal which can be interpreted by the external testerdevice 9.

The measurement converter circuit 3 may, by way of example, alsodetermine the phase angle of the first and second internal signals S1,S2, a periodic signal whose duration of a state determines the phaseangle of the first and second internal signals with respect to oneanother being output as a measured value. The determination of the phaseangle of two internal signals is complicated since the internal signalswould have to be read out into an external tester device 9, it beingnecessary to take account of signal delays on account of lead lengths.Furthermore, the internal signals would be influenced considerably bythe load of the leads, so that a correct phase angle would not be ableto be determined.

FIG. 2 illustrates an activation circuit in accordance with anembodiment of the invention. The activation unit 4 has an SR flip-flop20 with two NAND gates, which is connected by a set input S to theconnection pad 6. The output A of the SR flip-flop 20 is connected to acontrol input of a field-effect transistor 21. A first connection of thefield-effect transistor 21 is likewise connected to the connection pad6, a second connection of the field-effect transistor 21 is connected toa predetermined potential, preferably a ground potential GND. The outputA of the SR flip-flop 20 is connected to the switch-on line 8 via afirst inverter 22, the switch-on signal EN being present at an output ofthe first inverter 22. A reset input R of the SR flip-flop 20 isconnected in such a way that a reset signal PWRON can be applied inorder to reset the SR flip-flop 20.

After the switch-on or after an initialization of the integratedcircuit, the SR flip-flop 20 is initially not set. The node A is then ata high potential. The field-effect transistor 21 is thus activated, i.e.it has a very low resistance. As a result, the connection pad 6 ispulled to the ground potential. This prevents the connection pad 6,which is otherwise not electrically connected, from being able to float,i.e. from being able to assume an undefined voltage as a result ofcharge flows in the integrated circuit. This prevents a high potential,which sets the SR flip-flop 20, from inadvertently being applied to theset input of the SR flip-flop 20.

If a voltage pulse which—taking account of the setup and hold time—setsthe SR flip-flop 20 is applied to the connection pad 6, then a lowpotential is permanently present at the output A and turns off thefield-effect transistor 21, i.e. the latter has a very high resistance.The connection pad 6 is thus isolated from the ground potential GND. Bymeans of the set SR flip-flop 20, a high level of the switch-on signalEN is applied to the switch-on line 8, said signal activating themeasurement converter circuit 3. The measurement converter circuit 3remains activated and the field-effect transistor 21 remains turned offuntil a reset signal PWRON resets the SR flip-flop 20. The driver powerfor the application of the activation signal must be chosen in such away that, with respect to the fixed potential applied via the resistor11, the activation signal applies a high level at the set input S of theSR flip-flop 20.

After the SR flip-flop 20 has been set by the application of the highpulse of the activation signal at the connection pad 6, a measured valueM can thus be tapped off from the measurement converter circuit 3 viathe connection pad 6. The voltage levels of the measured value M do notinfluence the SR flip-flop 20 since the SR flip-flop 20 cannot be resetvia the set input S.

FIG. 3 illustrates a possible measurement converter circuit 3. Themeasurement converter circuit 3 has a switching device 23, whichforwards the first and the second internal signal S1, S2, via respectiveNAND gates 25, 26, to a phase comparison unit 24. The switching device23 applies the internal signals S1, S2 to the phase comparison unit 24only when the switch-on signal from the switch-on line 8 indicates anactivation of the measurement converter circuit 3. The switching device23 has a first NAND gate, to whose first input the switch-on signal ENis applied and to whose second input the first internal signal S1 isapplied. The switching device 23 has a second NAND gate 26, to whosefirst input the switch-on signal EN is applied and to whose second inputthe second internal signal S2 is applied.

The phase comparison circuit 24 receives the signal at the output of thefirst NAND gate 25 and applies it via a transmission gate 27 to a firstinput of a third NAND gate 28. The signal at the output of the secondNAND gate 26 is applied via a third inverter 29 to a second input of thethird NAND gate 28. The output of the third NAND gate 28 is connected toa control input of a second field-effect transistor 30. The secondfield-effect transistor 30 is or is not activated in accordance with theoutput of the third NAND gate 28. The second field-effect transistor 30is connected by a first connection to the connection pad 6 and by asecond connection to a predetermined potential, preferably a groundpotential.

Application of a voltage at the connection pad 6 by the external testerdevice 9 can bring about a current flow in the second field-effecttransistor 30, which varies depending on the phase angle of the firstand second internal signals. Thus, a current flows if the secondfield-effect transistor 30 is activated, and the current flow isinterrupted if the second field-effect transistor is interrupted. Thetransmission gate 27 is provided in order to hold the delay of thesignal at the output of the first NAND gate 25 up to the first input ofthe third NAND gate 28 essentially at the same value as the delay of thesignal at the output of the second NAND gate 26 via the inverter 29 upto the second input of the third NAND gate 28.

FIG. 4 illustrates the signal profile of the first and second internalsignals S1, S2, and also the output current profile in the connectionpad 6. It can be seen that a current flows apart from the time periodbetween the rising edge of the first internal signal S1 and the risingedge of the second internal signal S2. Thus, the length of the currentflow interruption then indicates the phase angle of the first and secondinternal signals. Through the interconnect to the connection pad 6, thelength of the lead between the tester device 9 and the connection pad 6and also through the contact-making of the connection pad 6 with the aidof a measuring tip, there results a capacitance or inductance whichsmoothes the current profile through the connection pad 6. The result isa measurement voltage whose magnitude essentially depends on the phaseangle of the first internal signal S1 and of the second internal signalS2.

The measurement converter circuit 3 is not restricted to a circuit formeasuring the phase angle between two signals. In principle, anyinternal signal can be conditioned by means of the measurement convertercircuit 3 in such a way that the internal signal can be checked in asimple manner by means of the external test device 9. The measured valuemay be transmitted to the external measuring device for example as acurrent, voltage or frequency. It is also possible to output a digitaloutput signal in the form of a high or low potential to the externaltester device 9.

1. A test circuit of an integrated circuit, the test circuit comprising:a connection pad; a measurement converter circuit connected to theconnection pad and configured for converting one or morecircuit-internal signals into a measured value; and an activation unitconnected to the connection pad and configured for activating themeasurement converter circuit in response to an activation signalreceived via the connection pad; the activation unit being configured insuch a way as to permanently switch on the measurement converter circuitin response to the activation signal, it then being possible to tap offthe measured value via the connection pad.
 2. The test circuit of claim1, wherein the activation unit comprises: an SR flip-flop; and aswitching element which, with the SR flip-flop not set, is switched toconnect the connection pad to a predetermined potential via a definedresistor and which, with the SR flip-flop set, is switched to isolatethe connection pad from the predetermined potential.
 3. The test circuitof claim 2, wherein the SR flip-flop is set in response to theactivation signal.
 4. The test circuit of claim 2, wherein theconnection pad is connected to a set input of the SR flip-flop in orderto set the SR flip-flop in response to the activation signal.
 5. Thetest circuit of claim 2, wherein the switching element comprises afield-effect transistor.
 6. The test circuit of claim 2, furthercomprising a reset input of the SR flip-flop configured to receive oneor more signals to reset the SR flip-flop and to deactivate themeasurement converter circuit.
 7. The test circuit of claim 1, whereinthe measurement converter circuit comprises a phase comparison unit,which compares the phase angle of a first and a second periodic signalwith one another and outputs a pulse-width-modulated signal depending onthe phase angle.
 8. The test circuit of claim 1, wherein the measurementconverter circuit comprises a digital-to-analog converter unitconfigured to convert a digital signal into an analog voltage valuecapable of being output as the measured value.
 9. A test circuit of anintegrated circuit, the test circuit comprising: a measurement convertercircuit configured for converting one or more circuit-internal signalsinto a measured value and outputting the measured value for receipt byan external testing device; an activation unit configured for activatingthe measurement converter circuit in response to an activation signal; aconnection pad to which the measurement converter circuit and activationunit are connected; and a switching element which is switchable toconnect the connection pad to a predetermined potential and switchableto isolate the connection pad from the predetermined potential.
 10. Atest circuit of an inteprated circuit, the test circuit comprising: ameasurement converter circuit configured for converting one or morecircuit-internal signals into a measured value and outputting themeasured value for receipt by an external testing device; wherein themeasurement converter circuit comprises a phase comparison unit, whichcompares the phase angle of a first and a second periodic signal withone another and outputs a pulse-width-modulated signal depending on thephase; and an activation unit configured for activating the measurementconverter circuit in response to an activation signal.
 11. An apparatus,comprising: a test circuit of an integrated circuit, the test circuitcomprising: a measurement converter circuit configured for convertingone or more circuit-internal signals into a measured value; and anactivation unit configured for activating the measurement convertercircuit in response to an activation signal; and a test systemcomprising a tester device connected to the integrated circuit via atester channel in order to transmit the activation signal to theactivation unit and, after the transmission of the activation signal, toreceive the measured value.
 12. The apparatus of claim 11, wherein thetester device is configured to compare the received measured value witha desired measured value in order to check a function of the integratedcircuit.
 13. The apparatus of claim 11, wherein the measurementconverter circuit comprises a phase comparison unit, which compares thephase angle of a first and a second periodic signal with one another andoutputs a pulse-width-modulated signal depending on the phase angle. 14.The apparatus of claim 11, wherein the test circuit comprises aconnection pad to which the measurement converter circuit, theactivation unit and the tester device are connected, whereby theactivation unit receives the activation signal via the connection padand the measurement converter circuit outputs the measured value to theconnection pad.
 15. The apparatus of claim 14, wherein the test circuitfurther comprises a switching element which is switchable to connect theconnection pad to a predetermined potential and switchable to isolatethe connection pad from the predetermined potential.
 16. The testcircuit of claim 9, wherein the measurement converter circuit comprisesa phase comparison unit, which compares the phase angle of a first and asecond periodic signal with one another and outputs apulse-width-modulated signal depending on the phase angle.